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selected, the width and positioning of each band
being selectable by software.
2.4 Radiometric Resolution
The pushbroom principle of linear array
imagers offers greatly improved radiometric
responsitivity, compared to optical-mechanical
scanners. High radiometric sensitivity and
dynamic range allow small changes in features to
be observed. The dynamic range for all of the
channels of the MEIS II sensor, for example, is
better than 4500:1, and close to 7000:1 for four of
the channels (Till et al., 1987). The FLI and
CASI systems have dynamic ranges better than
4000:1 (Borstad et al., 1989).
This higher sensitivity to spectral intensity
with airborne linear array sensors is the result of
the incorporation of multiple detectors in an array,
of which each is able to integrate over one pixel
for the whole scan line. The integration period or
each pixel is approximately 3000 times longer than
optical-mechanical scanners allowing smaller
radiances to be measured due to improved noise
equivalent radiance (McColl et al, 1983). This
higher spectral sensitivity provided signal-to-noise
ratios greater than 100:1 over forested terrains
using the MEIS II system (Till et al., 1986a).
2.5 Geometric Performance
The superior geometric quality of the
output imagery from linear array systems, when
compared to equivalent optical-mechanical imagery,
allow such data to be more easily integrated with
geo-referenced computer-based inventories. The
MEIS airborne system, for example, provides inter
channel pixel registration to within 0.15 pixel, over
the complete range of aircraft altitudes.
For applications relating to cartography or
multitemporal registration, additional processing of
the data is required to correct for other changes in
aircraft motion such as changes in velocity, pitch
and yaw, etc. A processing system has been
developed to correct MEIS imagery using flight
navigation data. Based on algorithms developed
by Gibson (1984), aircraft inertial navigation data
are used to compute positions for all input pixels
and to resample the imagery to a user selectable
geo-referenced output grid.
Geo-coded imagery has been produced
from FLI imagery using Moniteq’s AIR-2 software
and flight parameters recorded simultaneously
with the imagery. Approximately one pixel
precision has been achieved with spatial mode
data (Buxton, 1988).
2.6 Stereo Capability
In addition to the nadir-mode of operation,
the MEIS system can be used to acquire
continuous fore-aft stereo imagery, by using the
precision stereo mirror module. In the stereo
mode, two of the eight channels look fore and aft
at angles of +/-35° while six channels look nadir.
The data may be combined with aircraft altitude
data to provide digital terrain information and
standard geometrically corrected products.
Fore-aft stereo acquisition may also be
able to provide information appropriate for forest
stand height determination. With geometric
correction capability stereo imagery is also suitable
for base mapping and topographic mapping.
2.7 Digital Format
The digital format of airborne linear array
imagery allows easier integration with digital
databases such as geographic information systems
and lends itself to automated processing of the
imagery. In a digital form the imagery can also
be more easily enhanced to improve visual
interpretation. Digital imagery can also be easily
mosaicked to provide large area coverage or ortho
photos.
3.0 CURRENT LINEAR ARRAY SYSTEMS
The discussion of available linear array
sensors will be restricted primarily to systems
developed in Canada which have application to
forestry. The discussion will not include systems
developed for monitoring the atmosphere or
systems developed primarily for ground based use
(e.g., non-imaging spectrometers or radiometers).
3.1 MEIS History and Description
The MEIS (Multispectral Electro-Optical
Imaging Sensor) is an airborne linear array sensor
system developed for the Canada Centre for
Remote Sensing (CCRS). The MEIS airborne
multispectral imager was the first solid state
linear array imager to be developed and provide
data to the remote sensing community (Zwick et
al, 1978). It was part of a multi-year Canadian
research and development initiative to evaluate
pushbroom imager technology and to develop
applications.
Development was started in the late
1970’s with the prototype, two channel, MEIS I
sensor. The sensor had two optical channels each
with a 512-element photodiode linear array. MEIS
I was successfully operated in the laboratory and
in the air, and provided a useful demonstration of
the pushbroom imager concept. Results were
significant enough to lead to the development of
the current eight channel MEIS II. This version
was developed under contract to CCRS by
MacDonald, Dettwiler and Associates Ltd. of
Vancouver, British Columbia. In the four year
period from 1983 to 1986, close to two hundred
missions were flown with MEIS II throughout
Canada, USA and Australia (Till et al., 1986b).
